1. Field of the Invention
The present invention relates to a technology for controlling or purifying exhaust emissions of an internal combustion engine, and more particularly to an exhaust emission control apparatus including a particulate filter and an NOx occlusive agent.
2. Description of the Related Art
In recent years, it is desired that an internal combustion engine installed on a motor vehicle or the like should reduce the amounts of particulate matter (PM), nitrogen oxides (NOx), etc., discharged into the atmosphere.
To such a demand, in the past, for example, there has been proposed a technology that there are disposed in an exhaust passage of an internal combustion engine a particulate filter for collecting PM in an exhaust and an NOx catalyst adapted to collect NOx in the exhaust when the oxygen concentration of the exhaust flowing into the NOx catalyst is high and to discharge the NOx collected therein so as to be reduced when the oxygen concentration of the exhaust flowing into the NOx catalyst is low (for example, see a first patent document: Japanese patent No. 2722987, a second patent document: Japanese patent application laid-open No. 2001-303980, a third patent document: Japanese patent application laid-open No. H09-53442, and a fourth patent document: Japanese patent application laid-open No. H08-200049, and a fifth patent document: Japanese patent application laid-open No. 2000-240428).
Since there is a limit to the amount of PM that can be collected by a particulate filter, it is necessary to properly remove the PM collected by the particulate filter thereby to recover the PM collection capability of the particulate filter.
As a method of removing the PM collected by the particulate filter, it is general to combust the PM collected by the particulate filter by raising the temperature of the particulate filter.
When the temperature of the particulate filter is raised in the exhaust emission control apparatus including the particulate filter and the NOx catalyst, however, not only the temperature of the particulate filter but also the temperature of the NOx catalyst might rise in some cases. As a result, there might be the case that the NOx occluded in the NOx catalyst is discharged into the atmosphere without being purified.
In addition, the NOx catalyst occludes not only the NOx contained in the exhaust but also sulfur oxides (SOx) therein, so when the amount of occlusion of the sulfur oxides increases, there takes place so-called sulfur oxide poisoning in which the NOx occlusion capability of the NOx catalyst is reduces.
As a method of removing the sulfur oxide poisoning of the NOx catalyst, it is general to reduce SOx to sulfur dioxide (SO2) while thermally decomposing the sulfur oxides from the NOx catalyst by raising the temperature of the NOx catalyst. However, when the temperature of the NOx catalyst is raised so as to remove the sulfur oxide poisoning, the NOx occluded in the NOx catalyst might sometimes be discharged into the atmospheric without being purified.